The long-term objective of this study is to develop appropriate and effective treatment for people with dual diagnoses of alcohol or other drug abuse and chronic mental illness. Dual diagnosis patients are particularly prone to disruptive, disinhibited, noncompliant behaviors and treatment failure. A high proportion are homeless, and others have unstable housing or are at risk for homelessness. These patients are also prone to institutionalization in mental hospitals and jails. The first specific aim of this study is to evaluate the clinical effectiveness of a special treatment intervention, termed continuous treatment teams, that will provide intensive case management and integrated alcohol, drug abuse, and mental health services for dual diagnosis patients. Treatment will specifically aim to decrease alcohol and drug abuse, housing instability and homelessness, and institutionalization. A second aim is to determine whether good outcomes in these primary domains will be associated with more general improvements in functioning. A third aim is to clarify pathways into and out of homelessness in this highly vulnerable population. The study design is a randomized clinical trial. A total of 300 consenting dual diagnosis patients will be randomly assigned to either intensive case management and integrated treatment through continuous treatment teams or to standard case management and referral for alcohol and drug abuse treatment through community support programs. These patients will all have chronic mental illness; approximately 90% will have alcohol abuse or dependence; and approximately 40% will abuse or be dependent on other drugs. Approximately 40% will have experienced recent homelessness, and an additional 25% will have experienced recent housing instability. Patients will be interviewed by a researcher at baseline and at follow-up intervals of six months over a period of three years. Laboratory monitoring of alcohol and drug abuse and medication compliance will be performed at baseline and every three months for three years. Evaluations will focus on alcohol and other drug abuse, housing instability and homelessness, and institutionalization in hospitals and jails. Other measures include psychiatric symptoms, functional status, and quality of life. Previous experience with this population in New Hampshire suggests an annual attrition rate of approximately 5%, yielding a final sample of at least 250.